Where Are the Baryons? III: Non-Equilibrium Effects and Observables

Numerical simulations of the intergalactic medium have shown that at the present epoch a significant fraction (40−50%) of the baryonic component should be found in the (T ∼ 10K) Warm-Hot Intergalactic Medium (WHIM) with several recent observational lines of evidence indicating the validity of the prediction. We here recompute the evolution of the WHIM with the following major improvements: (1) galactic superwind feedback processes from galaxy/star formation are explicitly included; (2) major metal species (O V to O IX) are computed explicitly in a non-Equilibrium way; (3) mass and spatial dynamic ranges are larger by a factor of 8 and 2, respectively, than in our previous simulations. We find: (1) non-equilibrium calculations produce significantly different results from ionization equilibrium calculations. (2) The abundance of O VI absorption lines based on non-equilibrium simulations with galactic superwinds is in remarkably good agreement with latest observations, implying the validity of our model, while the predicted abundances for O VII and O VIII absorption lines appear to be lower than observed but the observational errorbars are currently very large. The expected abundances for O VI (as well as Lyα ), O VII and O VIII absorption systems are in the range 50 − 100 per unit redshift at EW = 1km/s decreasing to 10 − 20 per unit redshift at EW = 10km/s. The number of O VI absorption lines with EW > 100km/s is very small, while there are about 1− 3 lines per unit redshift for O VII and O VIII absorption lines at EW = 100km/s. (3) Emission lines, primarily O VI and Lyα in the UV and O VII and O VIII in the soft X-rays are potentially observable by future missions and they provide complimentary probes of the WHIM in often different domains of the temperature-density-metallicity phase space as well as in different spatial locations; however, sometimes they overlap spatially and in phase space, making